m abbr. (1) meter; (2) minim.
m- abbr. Milli- (one-thousandth).
μ- abbr. Micro- (one-millionth).
M abbr. (1) methionine; (2) molar (mol/L); (3) Roman numeral for one thousand.
M- abbr. Mega- (one million).
mAb abbr. Monoclonal antibody.
macerate /MASS-er-ate/ v. To soften and break down into smaller parts — maceration /mass-er-RAY-shən/
Online Biology Dictionary (MACRO-)
macrocephalic /MACK-rō-sə-FAL-ick/ adj. Having an abnormally large braincase — macrocephalic /MACK-rō-SEF-ə-lee/ n.
macroevolution /MACK-rō-EH-və-LOO-shən/ n. The production during the course of evolution of new forms of life treated as distinct species.
macrofossil (also megafossil) /MACK-rō-FAW-səl/ n. A fossil large enough to be inspected with the naked eye. Compare: microfossil
macromolecule /MACK-rō-MAWL-ə-kyool/ n. A large, complex molecule such as DNA, RNA or proteins.
macroscopic /mack-rə-SKAWP-ick/ Visible to the naked eye.
macrophage A type of leukocyte produced by the division of monocytes, that protect the body by engulfing bacteria and debris. PICTURE OF A MACROPHAGE FORMING PROCESSES TO PHAGOCYTOSE TWO SMALLER PARTICLES
macropodid /mac-KRAW-pə-dəd/ n. A member of the mammalian family Macropodidae (/mac-krə-PAWD-də-dee/), which includes the kangaroos, wallabees, and wallaroos. ABOUT THE DIET OF KANGAROOS
magma /MAG-mə/ n. Molten silicate materials beneath the Earth's crust. Igneous rock is formed from cooling magma.
magnetic nanoparticles (mNPs) Microscopic particles manipulated with magnetic fields. Composed of magnetic elements (i.e., iron, nickel and cobalt) or compounds of such elements, mNPs are usually less than 1 micrometer wide. MORE INFORMATION
major histocompatibility complex (MHC) /hist-ə-cəm-PAT-ə-bill-ə-tee/ n. A region composed of many separate genes that control the immune cells. In humans it is found on chromosome 6. The MHC is part of the major immunogene complex.
major immunogene complex (MIC) /im-MYOON-ə-jeen/ n. A region composed of loci encoding histocompatibility antigens, lymphocyte surface antigens, proteins of the complement system, and immune response gene products. It includes the major histocompatibility complex (MHC).
malaria /mə-LAIR-ee-ə/ n. An infectious disease caused by sporozoans of the genus Plasmodium. Transmitted to humans by mosquitoes, the various forms of malaria result in more deaths worldwide than any other parasitic disease.
malignant /mə-LIG-nənt/ adj. Cancerous. A malignant tumor can invade nearby tissue and spread elsewhere in the body
Malpighian tubules /mal-PIG-ee-ən/ insect excretory organs that function in osmoregulation and cleanse the blood of nitrogenous wastes. They empty into the digestive tract.
mammal /MAM-əl/ n. A member of the vertebrate class Mammalia. Mammals nurse their young with milk.
mammalian /mə-MAWL-ee-ən, mam-/ Of or pertaining to mammals.
mammalogy /mam-MAWL-ə-jee/ n. The study of mammals — mammalogist /mam-MAWL-ə-jəst, -jist/
manatees /MAN-ə-teez/ n. Large herbivorous aquatic mammals belonging to the genus Trichechus of the order Sirenia. There is a saltwater form, the West Indian Manatee (T. manatus), which occurs in the Caribbean, and two freshwater forms the Amazonian Manatee (T. inunguis) and the West African Manatee (T. senegalensis), which inhabit the Amazon Basin and the rivers of western Africa, respectively. PICTURE (T. manatus)
Man abbr. Mannose.
ManN abbr. Mannosamine.
mandible n. /MAN-də-bəl/ n. The bone of the lower jaw.
mannosamine (ManN) A hexosamine derived from mannose. MOLECULAR STRUCTURE
mannose (Man) /MAN-ose/ n. An aldohexose sugar. MOLECULAR STRUCTURE
marine /mə-REEN/ adj. Of, pertaining to, or living in the sea.
marrow /MAIR-ō/ n. Flexible tissue present in the hollows of bones; new blood cells are produced in the marrow.
marsupial /mar-SOUP-ee-əl/ n. A member of the major division Marsupialia of Class Mammalia. A marsupium is present in the females of many, but not all marsupials.
marsupium /mar-SOUP-ee-əm/ n. (1) A fold of skin or pouch, containing the mammary glands, and serving to hold and protect the young during early development; found in many, but not all animals classified as marsupials; (2) a pouch present in certain non-mammals, such as fish, in which eggs are carried externally.
mass extinctions n. Events, observed at certain points in the fossil record, in which many different forms of life simultaneously cease to exist.
mass spectrometer /speck-TRAWM-ə-ter/ n. An instrument that determines the chemical compound's composition on the basis of the mass-to-charge ratio of charged particles (used in DNA sequencing).
mastology /mast-TAWL-ə-jee/ n. The scientific study of the breasts.
Mauer Jaw n. A lower jawbone found in Mauer, Germany in 1907; the type specimen of Homo heidelbergensis. PICTURE
maxilla /MACK-sil-ə/ n. A jawbone, especially of the upper jaw.
maxillary /MACK-sə-lair-ee/ adj. Pertaining to the jawbone, especially of the upper jaw.
Mb abbr. (1) megabase; (2) myoglobin.
meatus /mee-AID-əs, -ATE-əs/ n. A natural body opening, canal, or passage.
Online Biology Dictionary (MEGA-)
megabase (Mb) /MEG-ə-base/ n. A nucleotide sequence 1,000,000 bases in length.
megadont /MEG-ə-dawnt/ adj. Having large teeth.
megafossil /MEG-ə-FAW-səl/ n. See: macrofossil.
Megalobatrachus /MEG-ə-low-BAT-rə-kəs/ n. A synonym of Andrias.
megalocephaly /MEG-ə-low-CEF-ə-lee/ n. A condition in which the skull is abnormally large.
megaspores /MEG-ə-spore/ n. In the ovule of a flower, the four haploid cells produced by meiosis. Normally, only one goes on to produce a female gametophyte; the others degenerate.
Online Biology Dictionary (MEIO-)
meiosis /my-Ō-səs/ n. The process of two consecutive cell divisions that produces haploid sex cells and spores from diploid progenitor cells. Meiosis results in four daughter cells, each with a haploid set of unreplicated chromosomes. DETAILED DESCRIPTION OF MEIOSIS
meiospore /MY-ō-spor/ n. A spore produced by meiosis, which is therefore haploid and gives rise by division to haploid daughter cells, or to a haploid individual such as the gametophyte of a seed plant.
Online Biology Dictionary (MELAN-)
melanin /MEL-ə-nən, -nin/ n. A group of tyrosine-derived compounds found in eukaryotes. Variation in melanin genes has its most evident effect in color variation. Eumelanin, which is brownish black, is the most common form. Also common is pheomelanin, which is reddish brown.
melanistic /MEL-ə-NIS-tick/ adj. Dark-colored (due to the presence of higher levels of melanin than those seen in ordinary specimens).
melanosomes /mel-LAN-ə-sōms/ n. Melanin-containing organelles present in eukaryotic cells.
membrane /MEM-brane/ n. (1) A thin, flexible layer of tissue covering a bodily structure, or separating one structure from another; (2) the limiting surface of cellular cytoplasm, or of a cellular organelle; (3) a thin, flexible partition or covering of any type.
membrane potential n. Difference in charge between the cytoplasm and the fluid outside the cell; due to differences in the distribution of ions within and outside the cell.
menarche /mehn-AR-kee/ Onset of menses.
meninges /meh-NIN-jeez/ n. The membranes enclosing the spinal cord and brain.
meningitis /men-in-JYE-təs/ Inflammation of the meninges.
meniscus /mə-NIS-kəs/ (pl menisci /-ə-SKĪ, -ə-SĪ/ meniscuses /mə-NIS-kə-səs/) n. Any of the tough, slick pads of tissue filling the joint-space between bones.
menses /MEHN-seez/ n. The monthly bloody discharge occurring in women after puberty.
Online Biology Dictionary (MERI-)
meristem (often meristems) /MAIR-ə-stem/ n. Plant cells that remain embryonic, and that are therefore mitotically active, allowing plants to go on growing indefinitely as long as they live ("indeterminate growth").
meristematic /MAIR-ə-stəm-MAT-ik/ adj. Of or relating to meristem.
meristic variation /mer-IST-ik/ n. Discrete variation that can be counted, as number of bristles, fingers, or body segments.
Online Biology Dictionary (MERO-)
merogenesis /MAIR-ə-JEN-ə-səs/ n. Reproduction by segmentation.
merogony (also schizogony) /mair-AWG-ə-nee/ n. A mode of asexual reproduction seen in certain protozoan parasites. In merogony, the feeding-stage form (trophozoite) of the parasite increases in size while its nucleus and other organelles divide repeatedly without cytokinesis. Then this enlarged, reproducing form (schizont) ruptures, releasing merozoites. This mode of reproduction is characteristic of apicomplexan parasites.
meropia /mair-Ō-pee-ə/ Partial blindness.
merozoite /MAIR-ō-ZŌ-ite/ n. In the life cycle of certain protozoan parasites, a stage produced by merogony.
mesenteries /MEZ-ehn-ter-eez/ n. Organ-suspending membranes found in the body cavities of vertebrates.
Online Biology Dictionary (MESO-)
mesoderm /MEZ-ə-durm/ n. An early embryo's middle cell layer; develops into the muscles, skeleton, kidneys, spleen, gonads, and much of the circulatory system.
mesoglea (also mesogloea) /MEZ-ō-GLEE-ə/ n. The jelly separating the body walls of cnidarians and sponges.
mesoileum /mez-ō-IL-ee-əm/ n. The mesentery supporting the ileum.
mesophyll /MEZ-ō-fill/ n. The mid-layer of a leaf between the upper and lower epidermises; portion of a leaf where photosynthesis occurs.
Mesozoic Era /MEZ-ə-ZŌ-ik/ n. The second of the three geologic eras of the Phanerozoic Eon. It lasted from 251.0-65.5 mya.
messenger RNA (mRNA) n. RNA that serves as a template for protein synthesis. See also: genetic code.
met abbr. Methionine.
Online Biology Dictionary (META-)
metabolic /met-ə-BAWL-ick/ adj. Of, pertaining to, or involving metabolism.
metabolic pathway /met-ə-BAWL-ick/ n. A series of chemical reactions catalyzed by enzymes, occurring within an organism. The product of each successive reaction is the substrate of the next.
metabolism /mə-TAB-əl-iz-əm/ n. The chemical processes, taken as a whole, occurring within an organism.
metabolite /mə-TAB-ə-lite/ n. A product of a metabolic pathway.
metabolize /mə-TAB-əl-ize/ v. To transform a chemical compound into some other compound(s) via a metabolic pathway.
metacarpal /met-ə-KARP-əl/ n. Pertaining to the bones within the palm of the hand (or of the equivalent region in an animal).
metamorphosis /met-ə-MORE-fə-səs/ n. The transformation of a larva into the adult form.
metaphase /MED-ə-phaze/ n. (1) The second stage of mitosis (MORE INFORMATION); (2) The second stage of meiosis I (metaphase I) or of meiosis II (metaphase II).
metastasis /mə-TAST-ə-səs/ n. Spread of cancer cells to new regions of the body — metastasize /mə-TAST-ə-size/
methionine (M or met) /mə-THĪ-ə-neen/ n. An essential amino acid. CODONS | MOLECULAR STRUCTURE
methyl group /METH-əl/ n. The radical CH₃.
MHC abbr. Major histocompatibility complex.
MIC abbr. Major immunogene complex.
mice n. Small rodents belonging to the genus Mus. They are major vectors of disease. DISEASES CARRIED BY MICE | ROTATING TOMOGRAPHIC IMAGE OF A MOUSE SKELETON
Online Biology Dictionary (MICRO-)
microbe /MIKE-robe/ n. A microorganism.
microbial /my-KROBE-ee-əl/ adj. Of or pertaining to microbes.
microbiology /MIKE-rō-buy-AWL-ə-jee/ n. A microorganism — microbiologist /MIKE-rō-buy-AWL-ə-jəst, -jist/
microbiome /MIKE-rō-BUY-ōm/ n. The aggregate of microorganisms residing in an organism or in a particular locale within an organism.
microfilaments /MIKE-rō-FILL-ə-mənts/ n. Strands of actin present in almost all eukaryotic cells. Microfilaments are present in undulipodia and cilia, and make up part of the cytoskeleton. They act alone or with myosin to bring about contraction of the cell.
microfossil /MIKE-rō-FOSS-əl/ n. A fossil too small to be inspected without the aid of a microscope (less than about 4mm in diameter). Examples are the tests of microorganisms and pollen grains. Compare: macrofossil.
microinjection /MIKE-rō-in-JECK-shən/ n. Injection of a DNA solution into a cell by means of a microcapillary pipet.
microorganisms /MIKE-rō-ORG-ə-nizz-əms/ n. Organisms too small to be seen with the naked eye.
microcephalic /MACK-rō-sə-FAL-ick/ adj. Having an abnormally small braincase — microcephaly /MĪK-rō-SEF-ə-lee/ n.
microplastic particles /MIKE-rō-PLAH-stick/ n. Microscopically small plastic particles created when plastic waste breaks down into ever smaller fragments during the course of chemical and physical aging processes. In an aquatic environment, due to their size, microplastics pose a risk of being eaten by small shrimps, fish larvae and other organisms at the bottom of the food chain.
microscopic /mike-rō-SKAWP-ick/ adj. Invisible to the naked eye, but visible through the use of a microscope.
microtome /MIKE-rə-tome/ n. An instrument for preparing thin slices of tissue.
microtrichium (pl microtrichia) /mī-krō-TRIK-ee-əm, pl mī-krō-TRIK-ee-ə/ n. One of the tiny external hairs of an insect.
microtubules /MIKE-rō-TOOB-yoolz/ n. Hollow strands of tubulin present in all eukaryotic cells where they are present in undulipodia and cilia, and make up part of the cytoskeleton. MORE INFORMATION
milkweed bug (Oncopeltus fasciatus) n. A black insect with red markings often cultivated for research purposes.
Online Biology Dictionary (MILLI-)
milliliter (ml) /MILL-ə-LEE-ter/ n. One-thousandth of a liter.
millimeter (mm) /MILL-ə-MEE-ter/ n. One-thousandth of a meter.
minim /MIN-əm/ n. One-sixteenth of a fluidram.
Miocene /MY-ə-seen/ n. An epoch of the Neogene. It lasted from 23.03 to 5.33 mya. GEOLOGIC TIME SCALE
miscible /MISS-ə-bəl/ adj. Capable of being mixed.
Mississippian Period /miss-ə-SIP-ee-ən/ n. See: Carboniferous Period.
mite n. Any of a wide variety of minute or, often, microscopic tick-like arachnids, many of which are parasitic on animals or plants. Many, too, are free living and feed on such materials as dust, mold, or food stuffs. Among those attacking animals are the sarcoptic mange mites (family Sarcoptidae), which burrow in the skin, and the demodectic mange mites (family Demodicidae), which live in or near the hair follicles of mammals, including humans. Certain mites are also known to aggravate allergic diseases such as hay fever, asthma, and eczema. Mites include all members of subclass Acari of class Arachnida, except the ticks. PICTURE OF HOUSE DUST MITE (Dermatophagoides pteronyssinus) | PICTURE OF MICROSCOPIC RUST MITE (Aceria anthocoptes)
mitochondria (sing mitochondrion) n. Organelles that are the site of respiration in eukaryotic cells.
mitochondrial DNA (mtDNA) n. The DNA found in mitochondria, the organelles that generate energy for the cell.
mitoinhibitory /MY-tō-in-HIB-ə-tore-ee/ adj. Inhibiting mitosis.
mitosis /my-TŌ-səs/ n. The process of eukaryotic cell division that produces daughter cells genetically identical to each other and to the parent cell. No change in chromosome number occurs. DETAILED DESCRIPTION OF MITOSIS
mitotic /my-TAWT-ik/ adj. Of or relating to mitosis.
ml abbr. Milliliter.
μl abbr. Microliter (10⁶ liter).
mm abbr. Millimeter.
μm abbr. Micrometer (10⁶ meter).
mM abbr. Millimolar (mmol/L).
mmol abbr. Millimole.
model organism n. An organism studied as representative, either with respect to the course of some disease that also occurs in humans, or with respect to the general category of organisms to which the organism belongs.
Online Biology Dictionary (MOL-)
mol abbr. Mole.
molar n. One of the grinding teeth at the rear of the jaw.
molarity /mole-AIR-ə-tee/ n. The number of moles of solute in a liter of solution.
mole /mōl/ n. The grams containing Avogadro's number of molecules of a substance; a mole of a substance has the same weight in grams as its molecular weight measured in daltons.
molecular biology n. The study of the formation, structure, and function of macromolecules found in living organisms, particularly nucleic acids and proteins.
molecular farming n. The development of transgenic animals to produce human proteins.
molecular genetics n. The study, at the molecular level, of the function and structure of genes.
Mollusca /mə-Lə-skə/ n. A very diverse phylum, including such animals as gastropods, bivalves, octopuses, squids, cuttlefishes, and the chambered nautilus (at right). The mollusks have marine, freshwater, and terrestrial representatives. PICTURE
mollusks (or molluscs) /MAWL-əsks/ n. Animals in the phylum Mollusca.
molt /MŌLT/ v. To cast off the outer covering. Birds molt old feathers once or twice a year. Retiles molt old skin, and arthropods cast off the entire cuticle. Mammals also molt hair, but the term shed is usually used in this case.
molt /MŌLT/ n. The casting off or shedding of an outer covering. See: molt v.
monandry /MAW-nan-dree/ n. The condition of having a single anthers.
Online Biology Dictionary (MONO-)
monoclonal antibody (Mab) /MAW-no-KLONE-əl/ n. An antibody obtained from a single clone of plasma cells.
monocot /MAWN-ə-kot/ n. Short for monocotyledon
monocotyledon /mawn-ə-kaut-əl-EE-dən/ n. An angiosperm with one cotyledon. COMPARISON OF MONOCOT AND DICOT SPROUTS
monocytes /MAW-nə-sights/ n. A type of leukocyte that functions in the immune system by migrating to tissues and then differentiating into macrophages and dendritic cells.
monoecious /mə-NEE-shəs/ adj. Having both pistillate and staminate flowers on the same individual, or having perfect flowers (possibly in conjunction with pistillate and/or staminate flowers).
monogenic disorder /mawn-ə-JEN-ick/ n. A disorder caused by mutation of a single gene.
monomer /MAWN-ə-mer/ n. An atom or single molecule that can join with other, like atoms or molecules to form a polymer.
monosaccharides (also monosaccharoses) /MAWN-ō-SACK-ə-rides/ n. Soluble simple sugars that cannot be broken down into simpler sugars of lower molecular weight; they are the simplest carbohydrates, acting alone or as monomers in disaccharides; the molecular formula of a monosaccharide is a multiple of CH₂O. Monosaccharides are unaffected by enzymes and are absorbed directly by the body without change.
monosomy n. Having one copy of a given chromosome instead of the two normally present in diploid cells. Compare trisomy.
monotreme /MAWN-ə-treem/ n. A member of the major division Monotremata (egg-laying mammals) of Class Mammalia, the other two being eutherian mammals and marsupials. Monotremata includes the platypus and the two echidnas.
montane /MAWN-tane/ adj. Occurring in the mountains.
morbid map adj. A diagram showing the chromosomal location of genes associated with disease.
Morgan unit n. A unit (named for Thomas Hunt Morgan) expressing the relative distance between loci on a chromosome. One morgan (M) equals a crossover value of 100%, a decimorgan, a value of 10%, and one centimorgan, a value of 1%. In humans, a centimorgan is approximately equivalent to a megabase.
Online Biology Dictionary (MORPH-)
morphology /more-FAWL-ə-jee/ n. (1) the structure and form of an organ or organism; (2) the scientific study of the structure and form of organisms — morphologist /more-FAWL-ə-jəst, -jist/
mortality rate /more-TAL-ə-tee/ (often shortened to: mortality) n. The proportion of individuals that die in a specified period of time or under a particular condition.
mortification n. See: necrosis.
motile /MŌ-təl/ Capable of movement (as a gamete or microorganism).
motility /mō-TILL-ə-tee/ n. The characteristic of being motile.
mouse See: mice.
mRNA abbr. See: messenger RNA.
MRSA /MER-sə/ abbr. Methicillin-resistant Staphylococcus aureus, a strain of Staphylococcus aureus with resistance to beta-lactam antibiotics and cephalosporins. Such strains are not more virulent than others, lacking antibiotic resistance, However, the resulting infections are harder to treat with standard antibiotics.
mtDNA abbr. See: mitochondrial DNA.
Online Biology Dictionary (MUC-)
mucigen n. The substance, present in mucous cells, that is converted into mucin upon extrusion from the cell.
mucin n. The glycoprotein that gives wet mucus its slippery texture. Mucin is also present in various other fluids that protect the body from abrasion.
mucosal /myoo-KŌ-səl/ adj. Producing mucus.
mucous /MYOO-kəs/ adj. Pertaining to or producing mucus.
mucus /MYOO-kəs/ n. A slimy fluid secreted by mucous membranes. In addition to water, it contains mucin, leukocytes, epithelial cells, and inorganic salts.
Online Biology Dictionary (MULTI-)
multicellular /məl-tee-SELL-yə-ler/ adj. Composed of multiple cells of various types; said of a single organism.
multicellularity /məl-tee-sell-yə-LAIR-ə-tee/ n. The state of being multicellular.
multigene family n. See: gene family.
multiplexing n. Using a laboratory approach that performs multiple sets of reactions in parallel (simultaneously), which greatly increases speed and throughput.
murine n. Relating to the family, Muridae, to which rats and mice belong.
Online Biology Dictionary (MUTA-)
mutagen n. An agent that causes mutation.
mutagenicity n. The capacity of a chemical or physical agent to cause mutations.
mutant n. Affected by or having a mutation.
mutation n. (1) a change in the structure of a nucleotide sequence, often, but not necessarily, in a gene; (2) the altered sequence or gene resulting from such a change; (3) a change in the karyotype (chromosomal mutation) (4) an individual or type having such a change in its genetic makeup, and any descendants of that individual that in which it also occurs.
mutualism n. A form of symbiosis in which both participants benefit. For example, a clown fish lives inside a sea anemone (see picture at right) and is protected by it. In return, it brings scraps to the anemone, and lures larger fish into the anemone's tentacles.
mV abbr. Millivolt.
mya abbr. Million years ago.
myalgia /my-AL-zhə/ n. Muscle pain.
Online Biology Dictionary (MYC-)
mycelium /MY-SEEL-ee-əm/ n. A network of filaments (hyphae) that is a major component of the typical fungus.
Mycobacterium n. A genus of bacteria including the causative agents of leprosy and tuberculosis.
mycology /mike-AWL-ə-jee/ n. The scientific study of fungi — mycologist /mike-AWL-ə-jəst, -jist/.
mycosis /my-KŌ-səs/ n. Any disease caused by a fungus.
Online Biology Dictionary (MYEL-)
myelin /MY-ə-lin/ n. A substance forming an outer layer, the "myelin sheath", normally found around a neuron. Myelin electrically insulates neurons and thus allows proper function of the nervous system.
myelocyte /MY-ə-lə-SIGHT/ n. The cells in marrow from which leukocytes are derived.
myelodysplasia /MY-ə-lō-dis-PLAZ-yə/ n. Malformation of the spinal cord.
myelogenic /MY-ə-lō-GEN-ik/ adj. (1) produced in the marrow; (2) producing marrow.
Online Biology Dictionary (MYO-)
myoglobin /MY-ə-GLŌ-bin/ n. A pigmented protein storing oxygen in muscle cells.
myosin /MY-ə-sin/ n. A protein that forms microfilaments in eukaryotic cells; interacts with actin to bring about cellular contraction; essential component of sarcomeres.